Which of these numbers is prime? ${13,\ 27,\ 56,\ 57,\ 81}$
Answer: A prime number has exactly two factors: 1 and itself. The factors of 13 are 1 and 13. The factors of 27 are 1, 3, 9, and 27. The factors of 56 are 1, 2, 4, 7, 8, 14, 28, and 56. The factors of 57 are 1, 3, 19, and 57. The factors of 81 are 1, 3, 9, 27, and 81. Thus, 13 is a prime number.